The engine of this type, i.e., a spark-ignition engine which adopts the plug cap of the type integral with the ignition coil (and allocates a corresponding ignition coil per ignition plug of every cylinder and take the electronically electric-distributing system for controlling the electric-distribution in correspondence with the cylinder's timing) has a long lifetime period and can alleviate the electric-wave noise originated from the electrical-energy distributor of high voltage. Further, it has a certain degree of freedom at the ignition timing and is more advantageous on the point of the optimum ignition-timing control than the distributor system and therefore is coming to lead the others.
For example, the plug cap of the type integral with the ignition coil the Patent Literature 1 discloses, as shown in FIG. 2, has such a structure that a plug is inserted into a deep plug hole intended for the ignition plug to be arranged between a pair of intake and exhaust valves opposite to each other. The ignition coil is placed on a recess for attachment, formed by concaving an upper surface of a cylinder head-cover downwardly and fixed thereto.
A shielding cylindrical member that forms the plug hole is fitted into an attaching hole of the cylinder head and has an upper end extending through the cylinder head-cover. In consequence, the plug cap can be attached in such a manner that it drops from above the head cover into the shielding cylindrical member.